The invention relates to automatic mooring winch systems.
It is known to use a mooring winch to bring a ship to a berth at which it is to be moored and then to hold the ship at the berth using the winch to perform automatic mooring duty. During the automatic duty, the winch is required to maintain tension in the mooring rope at all times. Wind and movements of water due to tide, current or other effect cause movements of the ship which can be resisted only up to the limits of safe tension in the mooring rope and the winch is set to pay out rope should the tension rise to the limit. The winch is arranged to recover rope as soon as the tension reduces so that at no time does the tension in the rope fall to zero.
At present, known winches used for automatic duty are either steam driven or, if electric, usually have DC drive motors. It is advantageous to eliminate maintainance of ships' deck equipment wherever possible. Steam winches entail maintainance of many mechanical parts and of insulation of long steam lines on deck. DC winches entail maintainance of Ward-Leonard drive equipment.
A mooring winch capable of automatic duty is known having an AC motor, but a load sensor is necessary and the motor and winch brake must be energised or de-energised repeatedly which leads to frequent maintainance work.
A mooring winch having an AC motor was proposed in U.S. Pat. No. No. 3,774,883 in which in a first system the winch motor is energised for automatic mooring duty from an AC source at a frequency of some 30 cycles per second. The system described is such that constant tension is maintained at extremely low speeds of motor operation only at or near stalled condition of the motor, which is energised continuously in the rope in-haul sense during automatic mooring duty.
This is emphasised in U.S. Pat. No. 3,774,883, which proposes a second system for towing applications in which fluctuations in tension have to be avoided. Clearly, when towing duty is undertaken it is unlikely that the winch motor can remain at or near its stalled condition and will be required to run at higher speeds.
In that second system therefore, it is proposed in U.S. Pat. specification to use a scope sensor which in response to rotation of the winch drum changes the resistance in the circuit of a DC current source supplying the rotor field of an alternator. The alternator has a stator winding at which a voltage is derived and fed to the winch motor.
Thus, as the sea conditions force the winch to pay out rope the scope sensor causes an increase in voltage to be fed from the alternator stator winding to the winch motor and the rope tension is increased, tending to haul rope in.
Clearly, even with this arrangement the rope tension is not maintained truly constant but fluctuates considerably, the greater the change in rope length, the greater the tension fluctuations will be.
The systems described in U.S. Pat. No. 3,774,883 employ in addition to the winch motor, a second AC induction motor which directly drives an alternator and which is also mechanically coupled to an AC squirrel-cage motor. The latter is driven idly during automatic mooring duty but is used to drive the second induction motor referred to above when high-speed in-haul of rope is required.
The system described in U.S. Pat. No. 3,774,883 is therefore extremely bulky and requires a great deal of mechanical maintainance. It does not make use of any electronic control circuitry to achieve a compact and seaworthy system.
Furthermore, the system requires additional cooling fans or fans to dissipate the heat generated in the resistance grid banks and in the second induction motor and the squirrel cage motor.
Motors other than for mooring winch applications are required to run in a given sense of rotation only and are energised in that sense only. Suitable energisation currents and voltages may be derived from a supply for such motors by what are generally known as "motor drives".
It has been proposed by D. W. Miller and R. G. Lawrence in "Electronics & Power" for October 1976 at pages 675 to 678 that such motor drives should be such as to produce quasi-square-wave outputs using rectification, chopper and inverter techniques or to produce synthesised sine waveform outputs using pulse width modulation techniques.
It should be noted that, as proposed by Miller and Lawrence, the motor duty required is solely drive motor duty, the motor being required to produce torque only in the sense of energisation. Furthermore, the motor has only a single duty to perform and each of the motor drive systems proposed gives an output which energises the motor throughout the entirety of its duty.
It should also be noted that in the case of quasi-square-wave outputs, Miller and Lawrence propose that thyristors in the inverter stage should each turn on for a period of 180 electrical degrees.
It should further be noted that Miller and Lawrence are especially concerned with speed control and they teach that a lower speeds motor frame size must be increased or forced cooling must be used to avoid excessive temperature rise in the motor.
However, Miller and Lawrence do not contemplate any application of inverter techniques beyond motor drive applications and in particular they do not contemplate or consider energisation of a motor in non-drive conditions i.e. in stalled non-rotary conditions; and the condition in which the motor is required to produce torque to resist motion of the load. The latter condition is not one in which the motor functions as a drive motor but rather one in which the motor is also obliged to resist the rotation impressed upon it and produces torque in a sense opposite to that in which it is being energised.
The invention goes beyond the proposals of Miller and Lawrence who consider the motor producing torque only in the same sense as that of energisation and only at speeds above some 5% of full rated synchronous speed. For example, above 75 revolutions per minute (see FIG. 4 of Miller and Lawrence reference).
By contrast I have found that inverter means may be successfully used to energise continuously an AC winch motor at zero speed (stalled stationary condition) and at low negative speeds, where the motor produces torque in a sense opposite to the sense of energisation and at low positive speeds so as to perform automatically the normal automatic mooring duty.
I have found that such continuous energisation enables the motor to produce adequate torque without excessive temperature rise. I have also found the motor response to changing conditions is excellent; that constant tension can readily be maintained in the rope paying out sense up to the highest speeds likely to be encountered during automatic mooring duties; and that rope recovery is readily catered for up to the highest speeds likely to be encountered during automatic mooring duties.